For many applications such as mobile phones, portable audio players or car audio systems etc., small loudspeakers are required to fit the small space available. Still, it is intended to be able to reproduce low frequency audio signals with such loudspeakers. A simple linear (i.e. signal level independent) equalizing to enhance the low frequency range of such small loudspeakers below its natural cut-off frequency often results in an unacceptable high electrical power in the low frequency region. This demands a large power amplifier, and the high power can result in distortion due to too large cone amplitudes of the loudspeaker diaphragm, or it may even result in permanent damage of the loudspeaker suspension or coil.
Some different non-linear signal processing methods have been proposed to pre-process an audio signal in order to enhance low frequency output from such small loudspeakers. Often such pre-processing is referred to as “bass enhancement” or “low frequency bandwidth extension”. Audio bandwidth extension is known in the field of audio and described thoroughly e.g. in the book “Audio Bandwidth Extension” by Erik Larsen and Ronald M. Aarts, John Wiley & Sons Ltd. 2004, ISBN 0-470-85864-8. As described in this book, low frequency bandwidth extension can be obtained by utilizing psychoacoustic properties of the human auditory system, i.e. to provide the listener with the perception of a larger amount of low frequency content than is physically present. Many of such approaches are based on introducing a non-linearity. E.g. synthesizing pure tones at frequencies well above the low cut-off frequency of the loudspeaker, wherein the pure tones are selected such that the human brain is “cheated” to perceive a low frequency tone which may not at all be physically present. On page 61 of the mentioned book, in a common approach it is mentioned as a requirement that the original signal spectrum is maintained at all signal levels, i.e. amplitude linearity.
U.S. Pat. No. 6,678,380 by Philips describes an audio system comprising a circuit for processing an audio signal, whereby the circuit comprises a harmonics generator coupled to the input for generating harmonics of the audio signal, and adding means coupled to the input as well as to the harmonics generator for supplying a sum of the audio signal and the generated harmonics to the output. It is claimed that an auditory illusion can be created by replacing low-frequency tones, by harmonics of these tones. Thus, in essence the harmonics are added so as to give the impression of bass tones that can not be reproduced by a small loudspeaker.
U.S. Pat. No. 5,359,665 by Aphex describes another bass enhancement system including a bass compressor having a variable gain amplifier controlling gain controlled by a signal level detector sensing the level of the bass components. In this way high bass amplification is provided at low bass levels, while less bass amplification is provided at higher bass levels. In general, compressors are used for reducing the dynamics of a signal. Thus the difference between the quiet parts and the loud parts is lessened, and thus the overall signal can be boosted. In order to optimise the compressor so-called attack and release times have to be set.
The mentioned approaches may result in acceptable sound quality in case of low frequency pure tones. However, since US 2003/0044023 A1 adds pure tones at higher frequencies in all cases, this can result in an unacceptable low sound quality due to highly audible intermodulation distortion. The compressor solution in U.S. Pat. No. 5,359,665 can result in audible “pumping” effects, due to the inherent problem of using attack- and release times, thereby providing a rather poor sound quality. Further, for application within low cost miniature equipment, the methods require too much signal processing power to be acceptable.